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Since there are only a few ways to connect the I/O components to the buses and the numbers of I/O ports for different systems are different, standard I/O chips are designed to provide standard interfaces to microprocessors and to allow the user to specify the number of input ports and output ports using a configuration program. I/O chips usually have a small set of internal switches (registers). Each switch has a unique address that allows the user to set or clear the individual switch.
Using I/O chips can reduce hardware design and development times as well as encourage reuse of the hardware in different applications. Since companion I/O chips are available for every microprocessor family, people usually do not design the I/O ports from scratch.
The 68HC12 microcontroller includes I/O configuration circuits. The user can write a short program to specify the functionality of each I/O pin.